Printed circuit boards (PCBs) usually include solder mask coatings applied to the copper traces for protection against oxidation, and to prevent solder bridges being generated between adjacent solder pads. The solder mask coating is formed by a solder mask ink, which includes a base agent and a hardening agent. The base agent comprises epoxy acrylate. The hardening agent comprises epoxy resin. The epoxy acrylate usually includes carboxyl groups (—COOH) so that the solder mask coating may be etched by an alkali solution during exposure and development. However, the epoxy groups of the epoxy resin may react with the carboxyl groups of the epoxy acrylate under a normal temperature, so that the solder mask ink must be stored at a temperature lower than 5 degrees Celsius, which increases the cost for storage.